←back to thread

Asynchrony is not concurrency

(kristoff.it)
289 points kristoff_it | 1 comments | 18 Jul 25 19:21 UTC | HN request time: 0s | source
Show context
threatofrain ◴[18 Jul 25 19:57 UTC] No.44609107[source]▶
IMO the author is mixed up on his definitions for concurrency.

https://lamport.azurewebsites.net/pubs/time-clocks.pdf

replies(4): >>44609321 #>>44609563 #>>44610163 #>>44611854 #
tines ◴[18 Jul 25 20:16 UTC] No.44609321[source]▶
Can you explain more instead of linking a paper? I felt like the definitions were alright.

> Asynchrony: the possibility for tasks to run out of order and still be correct.

> Concurrency: the ability of a system to progress multiple tasks at a time, be it via parallelism or task switching.

> Parallelism: the ability of a system to execute more than one task simultaneously at the physical level.

replies(9): >>44609334 #>>44609420 #>>44609491 #>>44609531 #>>44609532 #>>44609822 #>>44609915 #>>44610273 #>>44611918 #
1. sriram_malhar ◴[19 Jul 25 02:05 UTC] No.44611918[source]▶
The phrase "multiple tasks at a time" is ill-defined, according to Lamport, because whose clock are you trusting.

For lamport concurrent does not mean what it means to us colloquially or informally (like, "meanwhile"). Concurrency in Lamport's formal definition is only about order. If one task is dependent or is affected by another, then the first is ordered after the second one. Otherwise, they are deemed to be "concurrent", even if one happens years later or before.